Data rate increases are key goals in developing cellular system standards, such as LTE and HSPA. Higher data rates typically require larger system bandwidths, such as the 100 MHz bandwidth being considered for use in IMT-Advanced (a fourth-generation (4G) communications standard under development by the International Telecommunications Union or ITU. Finding large swaths of contiguous radio frequency spectrum is difficult, however, because the spectrum is a limited resource that is shared by many different operators, running a variety of communication system types.
Aggregating contiguous and non-contiguous spectrum yields larger “system” bandwidths, at least from a baseband perspective. Spectrum aggregation thus provides one mechanism for realizing system bandwidths sufficiently large for supporting dramatically higher maximum data rates in 4G, such as 1 Gbps and above. Aggregation also provides advantages in terms of allowing spectrum adaptations to suit the needs of a particular situation and geographic location.
Current cellular systems, such as LTE, can be evolved to use non-contiguous spectrum through the introduction of multi-carrier transmission. With multi-carrier LTE, several non-contiguous portions of radio frequency spectrum can be allocated, with each portion supporting a “legacy” LTE system. With this arrangement, a 4G access terminal receives data on the aggregate of two or more LTE carriers, transmitted at different frequencies, and possibly with different carrier bandwidths. Note that the term “LTE carrier” denotes the composite of a potentially large number of OFDMA sub carriers defined within a given OFDMA frequency bandwidth. Thus, two LTE carriers at different carrier frequencies means two different sets of OFDMA sub carriers positioned at different bands in the radio frequency spectrum.
While aggregation allows for large system bandwidths, it can complicate receiver design and operation. For example, a single receiver chain practically may not have sufficient bandwidth to receive all of the aggregated portions of radio frequency spectrum. This prospect is even more likely where the aggregated spectrum is non-contiguous. With the implementation of wireless communication receivers with multiple receiver chains, each being tunable to a different portion of the aggregated spectrum, the use of multiple receiver chains increases power consumption in the general case, and thus is undesirable for access devices where battery life is a chief performance concern. Such access devices include mobile terminals, such as cellular radiotelephones, PDAs, pagers, etc.
Thus, for mobile device battery management reasons, it is desirable to manage transmissions that minimize the number of active receiver chains needed at given mobile stations. A wireless communication system working according to that principle will try to collect all traffic to a specific user within a specific spectrum part. If there are multicast transmissions going on in a certain spectrum part, e.g., TV channels, then that part of the spectrum will be particularly attractive for users that simultaneously use TV and data traffic services. For the benefit of these users, the system may want to squeeze as much traffic as possible into that spectrum part.
However, at some point this squeezing will fail, and additional transmissions at one or more other parts of the spectrum are needed. Transmission of this additional data is more costly from an energy point of view, as the receiving UE must activate one or more additional receiver chains, or must otherwise configure itself for reception over a larger bandwidth, and thereby consume more power.